1. Field of the Invention
This invention is a method of improving the channel-hot-carrier (CHC) lifetime of Si MOS and particularly CMOS transistors that have a SiO.sub.2 gate insulator fabricated of SiO.sub.2 or related materials.
2. Brief Description of the Prior Art
During the operation of semiconductor devices, particularly Si MOS devices having a SiO.sub.2 gate insulator, CHC effects cause the performance of the MOS transistors to progressively degrade over time. Some parameters of the transistor, such as the drain voltage and the gate length, can be changed to at least partly reduce this problem and improve the reliability of the device. However, these changes typically degrade the initial performance of the transistor, such as saturation current and speed. It is known that a higher performance, more reliable transistor can be fabricated by reducing the intrinsic CHC degradation. It is also known that a deuterium and/or hydrogen anneal can reduce the intrinsic CHC degradation over time. Deuterium and/or hydrogen diffuses to the gate electrode insulator, which is typically SiO.sub.2, and becomes chemically attached to the dangling Si bonds at the Si/SiO.sub.2 interface. This helps to prevent the formation of interface traps during CHC stress conditions which degrade the transistor performance over time.
Deuterium is preferred over hydrogen since it is significantly more effective than hydrogen at preventing the formation of the interface traps during CHC stress, as taught by J. W. Lyding et al. in an article entitled "Reduction of hot electron degradation in metal oxide semiconductor transistors by deuterium processing" Applied Physics Letters, Vol. 68, No. 18, Apr. 29, 1996, pp. 2526-2528. Lyding et al. noted that replacing hydrogen with deuterium during the final wafer sintering process reduces hot electron degradation effects in metal oxide semiconductor transistors. The exact cause of this large isotope effect was not known. This substitution increased the CHC lifetime of the transistor by factors of 10 to 50, this being borne out by the applicant herein. Lyding et al. delivered the deuterium to the region of the gate oxide in an oven through thermal diffusion. At present, CMOS devices are usually annealed in a deuterium gas at constant temperature of 400 to 450 degrees C. for 30 to 60 minutes and then cooled. This gas may be pure deuterium or a deuterium/gas inert to the process, preferably nitrogen, mixture. However, since Lyding et al. did not anneal near the end of the process flow, it is possible that prior steps in the process flow introduced hydrogen into the device structure. Therefore, it may be that many of the dangling bond sites at the Si/SiO.sub.2 interface are already occupied by hydrogen and so cannot be occupied by the deuterium. While the CHC lifetime can be improved by introduction of deuterium into bonding sites, the deuterium can only bond at the appropriate sites if such sites are available. This pre-existing hydrogen will occupy some bonding sites at the gate oxide/Si substrate interface region and, therefore, will prevent the deuterium from occupying these bonding sites. Later, these hydrogen-occupied sites will degrade faster than the deuterium-occupied sites during CHC stress and therefore lower the CHC lifetime of the device compared to a device that has a larger percentage of sites occupied with deuterium. It further follows that even when bonding with deuterium does take place at available Si bonding sites, such bonds are or can be broken when later fabrication steps involve heating to a temperature above T.sub.d, the temperature at which the bond between the hydrogen and/or deuterium and the Si is broken, causing the hydrogen and/or deuterium to diffuse through the Si wafer. If more hydrogen is present at the interface and if both the hydrogen and deuterium are competing for available Si bonds, the hydrogen will occupy a significant number of Si bond locations and the above-described problem will be present.